JB, I assume that you've heard all the usual aviation jokes and stories perpetuated as real or as myths or whatever (such as the Qantas maintenance defects reports which started life as a USAF or US Navy one). But there was also some stories going around in aus.aviation. One related to an SR71 "cruising" at 80,000ft or thereabouts. It requested clearance to 60,000ft which LA Center responded, "if you can get to 60,000ft, then go for it". The wry reply, "roger, DESCENDING to 60,000ft".
Now, if that aircraft was in ATC controlled airspace, then wouldn't it have a transponder operating and a squawk code assigned? If so, surely the controller would've had all of the aircraft's data on his display.
Speaking of transponders, the info that we see on Flight Radar is generated from them, right? If so, is the aircraft's speed coming from that, or does ATC's equipment measure it independantly?